Fundamentals of Cytogenetics and The Techniques Used


Chromosomes are thin fibre like structure that forms the most important part of the genetic material and is mostly confined inside the nucleus in eukaryotic cells. During cell division chromatin condense to form rod like structures containing centromere and chromatids. Chromosomes are composed of DNA, histone proteins and non-histone proteins.

Humans have 23 pairs of chromosomes which contain genes that are responsible for a character any abnormality in these chromosomes may lead to genetic diseases. Cytogenetics is the field of biology that deals with the study of these chromosomes.

Cytogenetics and Its History

Cytogenetics can be defined as an interdisciplinary branch of biology that make use of cell biology and genetics to study the behaviour of chromosomes during cell division that is during mitosis and meiosis.

Chromosomes were discovered by Carl Nageli in 1842 in plant cells. They were discovered in animal cells by Walther Flemming, and he was the one who described the process of mitosis in 1882. Karyotyping and establishment of fact that genes are responsible for determination of characters was proved in 20th century. It was in 1956 when it was established that humans have 46 chromosomes by Painter.

Techniques Used in Cytogenetics

There are several techniques used in cytogenetics to study the behaviour of chromosomes. These techniques are as follows-

  • Karyotyping

  • G-banded chromosome technique

  • Fluorescence in situ hybridization


  • This was the first method to study the chromosomes. This study refers to the visualization and analysis of chromosomes at metaphase stage. At this stage of mitosis chromosomes align themselves at the centre of the nucleus and are clearly visible.

  • At metaphase chromosomes are banded with the help of two stains by trypsin and Leishman's stain. These stains together create a unique banding pattern on the chromosomes.

Giesma Banding or G-Banding Technique

  • It is the most commonly used banding technique that is used in cytogenetics and is used to visualise condensed chromosomes.

  • It presents the photographic image of the chromosomes which help in the identification of genetic or chromosomally linked diseases.

  • This banding method makes use of a stain called as the Geisma stain. The denser regions in a chromosome called as the heterochromatin region stains darker whereas the less dense region called the euchromatin region stains lighter.

  • The bands are then numbered on each arm of the chromosome starting from centromere up to telomere. Bands on the chromosomes can be detected and described precisely with the help of number system any deviation is an indicative of an underlying abnormality.

  • The number of bands are same in case of homologous chromosomes which eases their identification.

  • Some other staining techniques include reverse banding or R-banding, Q-banding and C-banding method. These all bands can be observed with the help of a microscope.

Fluorescence In-Situ Hybridization (FISH)

This technique is used to determine sequence variations directly from the chromosome and provides information regarding the location of alteration. It is an advanced version of cytogenetic analysis and employed for gene mapping, identification of major deletion or copy number variations, disease diagnosis, etc.

  • This method uses a fluorescently labelled probe which hybridizes with cytogenetic cell preparations.

  • After the hybridization of the probe with the chromosomes to be analysed they are visualized using a fluorescence microscope. This method can be used to study any alterations at gene level in cancer cells.

Applications of Cytogenetics

  • Chromosome banding is considered one of the greatest innovations in the field of cytogenetics. Q-banding method has been used in the detection of chromosomal aberrations leading to blood cancer which was very difficult earlier. Banding technique made the study of cancer arising from stem cell chromosomal rearrangement which was impossible to detect earlier. Giesma banding technique have been used for diagnostic cytogenetics.

  • Fluorescence in situ hybridization has been used for DNA sequence mapping on chromosomes. Earlier for this radio isotopic methods were used but since that method was impractical and time taking therefore it has been replaced by fluorescence techniques as they are faster and less dangerous. This method is also used for ordering locations of the genes along the chromosomes.

  • Flow cytometry is used to analyse chromosomes suspended in a fluid. This method collects each and every chromosome of a karyotype and is amplified using polymerase chain reaction, the specificity of which is determined by hybridization.

  • Cytogenetics led to the discovery of transposons by Barbara McClintock for which she was awarded the Nobel Prize.

  • Cytogenetics have been used in the diagnosis of genetically linked diseases like Down’s syndrome, Turner syndrome, Klinefelter syndrome, Edwards syndrome, etc.

  • The discovery of Philadelphia chromosome was possible because of cytogenetics. This Philadelphia chromosome is responsible for chronic myelogenous leukaemia.


Since it’s inception cytogenetics has revolutionized the field of diagnostic medicine. It helped in the diagnosis of several genetic disorders while baby is in mother’s womb. Advances in the field of cytogenetics lead to discovery of transposons. Banding pattern has allowed us to study chromosomal aberration that can lead to cancers. The interdisciplinary nature of cytogenetics has innumerable prospects in health and medicine in future also.

Updated on: 17-May-2023


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